Thermal insulation formed of glass fibers is used in a multitude of applications. One process of manufacturing fiberglass insulation is a centrifugal process in which molten glass is emitted through the apertures of a rapidly rotating spinner. The glass is attenuated by contact with blasts of hot gases, such as steam or combustion gases. The attenuated fibers are typically sprayed with a binder, such as a phenol-formaldehyde, phenol-urea or polyacrylic acid binder. The fibers are then typically collected on a moving conveyor and dried and the binder, if any, is cured to form a pelt. The pelt may then be cut into strips and packaged as rolls or batts of insulation. Alternatively, loose-fill insulation or blowing wool may be made by cutting the fibers into small pieces to form cubes or nodules that are compacted and packaged for shipment.
Loose-fill fibrous insulation or blowing wool may be blown into a cavity between the framing members of a wall or it may be blown into the attic of a structure to form a layer of insulation that conforms to the structure and fills the cavity. The loose-fill insulation provides a low cost installation technique.
The insulation value, or “R-value”, of insulation designates the resistance to the flow of thermal energy. The higher the R-value, the better the insulating properties of the subject materials. R-value is determined by the thickness (T) of the fibrous insulation and the insulation thermal conductivity (k) using equation 1.R=T/k  (1)
As can be derived from Equation 1 the R-value of an insulation is increased with increased thickness or with decreased k-value. The thermal conductivity is a measure of thermal conductivity of a particular material. Specifically, it is the measure of the amount of heat, in BTUs per hour, which will be transmitted through one square foot of material that is one inch thick to cause a temperature change of one degree Fahrenheit from one side of the material to the other. The SI unit for thermal conductivity is watts/meter/Kelvin. The lower the thermal conductivity for a material, the better it insulates. The thermal conductivity is dependent upon a number of variables including density, fiber diameter and glass composition. Increased pack density and reduced fiber diameter generally lead to lower thermal conductivities. In manufacturing a wool pack, the fiber diameter and pack density are controlled to yield the thermal conductivity required to give the necessary R-value at the specified product thickness.
Fiberglass insulation is manufactured from various raw materials combined in such proportions as to give the desired chemical composition. This proportion is termed the glass batch. This composition of the glass batch and the glass manufactured from it are commonly expressed in terms of percentages of the components expressed as oxides: typically SiO2, Al2O3, CaO, MgO, B2O3, Na2O, K2O, Fe2O3, and minor amounts of other oxides. The glass composition controls the viscosity, liquidus temperature, durability, and biosolubility of the glass. Other important characteristics of the composition are raw material cost and environmental impact.
Glass wool manufacturers have designed their glass compositions to optimize the infrared radiation absorption or scattering of the glass wool and thus decrease the k-value of the glass wool and increase the R-value. In the article “Influence Of The Chemical Composition Of Glass On Heat Transfer Of Glass Fiber Insulations In Relation To Their Morphology And Temperature Of Use”, C. Langlais et al., J. THERMAL INSUL. AND BLDG. ENVS., Vol 18, (1994), pp. 350-376, it was shown that wool pack thermal conductivity decreased with boron oxide addition to the glass up to about a B2O3 concentration of 6-7%. This phenomenon is termed the “Boron Oxide Effect.”
U.S. Pat. No. 5,932,499 discusses the impact of boron on borosilicate glasses and discloses glass compositions including, in weight percent, 50-60% SiO2, 2-6% Al2O3, 2-9% CaO, 1-7% MgO, 14-24% B2O3, 10-15% Na2O, and 0-3% K2O. In addition to these basic ingredients, the glasses may optionally contain from 0-4% TiO2, 0-4% ZrO2, 0-3% BaO, 0-4% ZnO, and 0-2% F2. Other optional ingredients include transition metal oxides, especially Fe2O3, which can be added to increase the absorption and refractive characteristics of the glass in the near infrared (1-4 μm) range. However, near infrared absorption and refractive characteristics are not necessary for commercial building insulation. The '499 patent states that the composition dependent optical constants, for example, higher refractive and absorptive indexes, influence the blocking of radiation heat transfer. Prior art glasses PA-1, PA-2, PA-3 and PA-4 (shown in Table 1) are set forth in the '499 Patent and are said to absorb or scatter heat radiation more effectively than glasses with lower refractive and absorptive indexes in this range. Among other compositional changes described in the '499 patent, it is asserted the amount of B2O3 is increased to increase these optical constants. PA-5 is a general description of a typical high boron glass composition used in the industry.
TABLE 1EXAMPLEPA-1PA-2PA-3PA-4PA-5SiO257.055.258.152.848.63Al2O34.94.824.065.089.54CaO6.655.355.115.611.02MgO4.73.933.114.680.95Na2O17.1411.714.710.84.85K2O1.110.910.90.57B2O38.5181420.123.90
Thus, prior art teaches that modification of the glass composition provides a third way, in addition to fiber diameter and pack density, to control the thermal conductivity of a wool pack: the thermal conductivity of the wool pack can be reduced by increasing the B2O3 of the glass fiber comprising the wool pack. Since the B2O3-containing raw materials are the most expensive components of the glass batch, increasing the B2O3 content increases the cost of the glass batch. This cost increase is offset by the pack density reduction made possible by the increase in B2O3. There thus exists an optimum B2O3 content which corresponds to that point at which these two competing effects yield a minimum overall production cost. For this reason, low-boron glasses are not typically used in the manufacture of wool insulation.
The specific chemistry used in generating the data for the High Boron (Pelt) and High Boron (Cubed) used in FIGS. 1 and 2 is set forth in TABLE 2.
TABLE 2OxideWt. %SiO248.63Al2O39.54CaO11.02MgO0.95B2O323.90Na2O4.85K2O0.57Fe2O30.13TiO20.04SrO0.37
Low Boron glasses for use as glass fiber reinforcements are disclosed in British Patent Specification No. 520,427 melt and form at higher temperatures, requiring operating conditions which could not be practically met. In addition, devitrification (crystallization) during fiber forming often occurred. For example, British Patent Specification No. 520,247 discloses glass compositions that are substantially alkaline-free which contain CaO, MgO, Al2O3, and SiO2, and that may be modified by the addition of B2O3, CaF2, P2O5, or a small amount of an alkali such as Na2O, K2O, or LiO2. However, these glasses are difficult to fiberize in a continuous fiber process at a forming temperature, at 2350 F. (1288 C.).
U.S. Pat. No. 4,542,106 to Sproull discloses boron- and fluorine-free glass fiber compositions for use in the manufacture of continuous strand processes rather than in rotary processes. In general, the glass compositions contain 58 to 60 percent SiO2, 11 to 13 percent Al2O3, 21 to 23 percent CaO, 2 to 4 percent MgO, and 1 to 5 percent TiO2. The glass fiber compositions may also contain alkali metal oxide and trace quantities of Fe2O3. The fibers disclosed by the '427 Patent and the '106 Patent are not used in insulation and thus the inventor is unconcerned with infrared radiation absorption or scattering and these factors, as well as R-value, are not measured.
To reduce the cost of manufacturing glass fibers for loose-fill insulation, and to reduce environmental impact without increasing production costs, there is a need in the art for improved glass compositions having low boron content.